Image data processing methods employing a video motion damper circuit (hereinafter referred to simply as “VMD circuit”) and video motion damper system (hereinafter referred to simply as “VMDS”) are known as methods for handling receive errors in a digital video receiver which receives digital broadcasting such as ISDB-T (Integrated Services Digital Broadcasting for Terrestrial). A VMD circuit and a VMDS are disclosed, for example, in Patent document 1 (Japanese Patent Kokai No. 2003-299034). The VMDS is capable of reducing visual discomfort due to interruptions in reproduced images accompanying the receive errors by supplying a mixture of video and still images using image data stored within the circuit upon occurrence of and recovery from the receive errors.
FIG. 1 illustrates a configuration of a conventional VMD circuit. In the figure, a frame memory 11 indicated by FIFOs 1 to 7 is a first-in first-out type memory. In all stages the FIFOs 1 to 7 of the frame memory are connected in series. The output of the frame memory of the previous stage is connected to the input of the frame memory of the next stage, thereby to form a single large FIFO-type memory unit. In addition, output taps are provided to extract image data at the respective outputs of the stages of the frame memory 11. Each of the FIFOs 1 to 7 of the frame memory stores successive digital image data sets received by the digital video receiver, for example, in field units or in frame units. The number of the stages of the FIFOs of the frame memory 11 is of course not limited to that of the circuit illustrated in FIG. 1.
A tap selection portion 12 is a circuit which selects/switches an output tap from/between the stages of the frame memory 11, and extracts the received digital image data from the selected output tap.
An error detection portion 13 is a circuit which receives various signals representing errors such as out-of-carrier-synchronization errors, packet errors, or data error rate which are detected by a front-end circuit, data reproducing circuit or similar (not shown) of the digital video receiver. The error detection portion 13 further generates a detection signal in response to a predetermined error state, and sends the detection signal to the VMDS control portion 14.
A VMDS control portion 14 is a circuit which controls tap switching of the tap switching portion 12 according to a prescribed algorithm of the VMDS, based on the detection signal from the error detection portion 13.
The VMD circuit 10 within the digital video receiver may be provided between a channel decoder circuit which is a block for decoding coded compressed data in accordance with a broadcast standard, and a source decoder circuit which is a block for decompressing the compressed data and converting the decompressed data into a baseband signal. Alternatively, the VMD circuit 10 may be provided between the source decoder circuit and a video encoder circuit which is a block for converting digital image signals into analog image signals (none of these circuits are shown).
VMDS processing by the VMD circuit 10 will now be explained.
First, when the receive error does not occur, the tap switching portion 12 selects the output tap of the FIFO 4 in the frame memory 11. An output from the FIFO 4 is supplied to each of later-stage circuits as image output data from the VMD circuit 10. The FIFO 4 corresponds to the fourth frame memory from the FIFO 1 of the first stage of the frame memory 11. Hence when one frame of image data is stored in the frame memory of each of the respective stages of the FIFOs 1 to 7, referring to an output data table as illustrated in FIG. 2, image data delayed from the current input image data by four frames is output from the VMD circuit 10.
The numbers in FIG. 2 represent frame numbers of the image data, and the number surrounded by broken line represents the frame number of image data output from the FIFO tap selected by the tap switching portion 12.
Next, operation of the VMD circuit 10 during receive error occurrence and recovery will now be explained with reference to FIG. 3.
As illustrated in FIG. 3, it is assumed that a receive error has occurred in the 12th frame of input image data, and that the VMDS control portion 14 has been notified of the error information by the error detection portion 13.
After the FIFO 4 outputs the eighth frame, the VMDS control portion 14 then issues an instruction to the tap switching circuit 12 to switch the output tap to the FIFO 5. Then, after the FIFO 5 outputs two frames of the eighth and ninth frames, the VMDS control portion 14 issues an instruction to the tap switching circuit 12 to the next output tap position.
In response, the tap switching circuit 12 switches the output tap, and the processing as illustrated in FIG. 3 continues until the 11th frame that is the final normal frame received before occurrence of the receive error is output from the FIFO 7. Processing of switching the output tap position of the frame memory upon occurrence of a receive error is called “video motion damp” (VMD) processing.
Thereafter it is assumed that recovery is performed from the receive error at the 18th frame of the input image data.
When all data that had been stored before the error occurrence is output (i.e., when the 11th frame is output from the FIFO 7), the VMDS control portion 14 issues to the tap switching circuit 12 an instruction to select the output tape position of the FIFO 1 storing the 18th frame which is the beginning of image data after the error recovery.
Thereafter, as illustrated in FIG. 3, the VMDS control portion 14 issues to the tap switching circuit 12 instructions to sequentially advance the output tap position upon each output of a prescribed number of frames. When the output tap position of the frame memory 11 reaches the FIFO 4, the VMDS control portion 14 then issues an instruction to halt the tap switching processing by the tap switching portion 12 and to fix the output tap position at the FIFO 4.
Processing of switching the output tap position of the frame memory 11 after recovery from a receive error is called recovery processing.
During the intervals of the above-described VMD processing and recovery processing, as illustrated in the output data table of FIG. 3, a mixture of a video output with the frame number increasing one by one and a still image output with the stationary frame number is output from the VMD circuit 10. Accordingly, compared with the case in which the outputs of reproduced images are merely still images in response to a receive error, visual discomfort can be reduced.
However, in the reception environment in which receive errors occur frequently, for example, the VMD processing and the recovery processing may be occur simultaneously, and processing burden on a microprocessor and an internal memory of a control portion of the VMD circuit 10 may be increased. Furthermore, as explained above, the VMD processing and the recovery processing both require substantially the same amount of processing time. Nevertheless, the VMD processing is essentially necessary function, and the recovery processing is no more than preparation for the VMD processing upon occurrence of the next error. During processing intervals of the both types of processing, motion in the reproduced images is unnatural compared with during reception of normal data. So it is preferable that a ratio of the still images output during processing time is low.